The present invention relates generally to digital audio systems and, more particularly, to a system for driving a conventional speaker with a digital signal for the production of speech.
It is known in the prior art to convert audio signals, such as voice or musical signals, to digital signals, such as a pulse coded modulation (PCM) signal, which may then be recorded or transmitted to a distant point for reproduction. Specifically, an analog audio signal is digitally sampled at a constant rate, commonly 11 KHz or some integer multiple, and a digital word is generated and stored or transmitted at each sampling, the digital word representing the polarity and magnitude of the analog audio signal at the time of sampling. The digital word is then converted back to analog and applied to a conventional speaker.
Conventional vibrating cone or diaphragm-type speakers or audio transducers are analog devices. Additionally, the speakers provided in commercially available, consumer oriented personal computer (PC) products typically are inexpensive, relatively low quality components to maintain the cost of the PC at a competitive level. Such low-cost speakers are well-suited for typical personal computer PC applications in which only single-frequency tones or game noises are produced. For tones such as the "bell" tone commonly utilized in personal computer PC applications, a pulse train is generated which turns the speaker on and off at the desired tone frequency. For game sounds, such as "crashes" and "gunshots", a random waveform centered about zero is digitally generated and infinitely clipped and applied to the speaker.
Typically, again as a cost-saving measure, personal computers do not include a digital-to-analog converter (DAC) and its associated circuitry. While the production of relatively simple sounds may be satisfactorily accomplished by applying a digital or clipped signal directly to a speaker, high quality, recognizable speech and other complex audio utilized by today's sophisticated computer games and other audio systems require the use of a DAC to produce acceptable audio.
U.S. Pat. No. 4,805,220 issued on Feb. 14, 1989 to Richard P. Sprague and Kevin R. Kachikian discloses an all-software speech generating system which applies a digital signal to a computer speaker to switch the speaker on and off at an ultrasonic carrier rate and which varies the speaker on/off duty cycle at audio frequencies according to the speech or sound to be produced. Speech is produced by modulating the duty cycle of a square-wave carrier signal in such a manner as to continuously vary the pulse length in accordance with the audio signal representing the desired speech to be produced. While the speech generating system of U.S. Pat. No. 4,805,220 produces acceptable speech without the use of a DAC, errors arising from the difference in speaker diaphragm position at various audio levels and in the full on or off positions are not compensated for. The speech quality and overall fidelity of the sound produced may be improved utilizing error compensation techniques.